msm: mdss: dsi: fix error handling when clk enable fails

When clock enable fails, the error handling logic should ensure
to remove the reference counts for the master controller clocks only
if the master controller is valid. Add the necessary checks to ensure
this so as to avoid any null pointer dereferences.

Change-Id: If2779c4febe19d882a8247872de6fac94f5b0ded
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>